Field of the Invention
This invention relates to a process for manufacturing terephthalic acid and more particularly, to a process for manufacturing and recovering the highly purified terephthalic acid, in accordance with the practice of this invention comprising the following procedures: alkali weight-reduction waste water discharged from weight-reduction process in a polyester textile dyeing complex is dissolved in water, adsorbed to remove impurities, and through acid-neutralization, terephthalic acid of this invention may be obtained.
"Alkali weight-reduction waste water" is a remaining reactant discharged in a weight reduction-processing stage which is designed to give the polyester textile a silky property, as well as to improve the dyeing capability by treating the polyester textile with alkali metal/earth metal hydroxide, and thereby causing a part of the textile to become depolymerization. Said alkali waste water contains a large amount of alkali metal/earth metal hydroxide, terephthalic acid alkali metal/earth metal salt and ethylene glycol. In the past, a considerable amount of alkali weight-reduction waste water, generated from the large industrial complex crowded with dyeing plants, has been discharged by a conventional waste water treatment. Besides that, the sludges neutralized with such strong acid as hydrochloric acid or sulfuric acid have been used for landfills or sea-abandonments, while the remaining reactants have been discharged by a conventional waste water treatment. Therefore, the alkali weight-reduction waste water generated from the polyester weight-reduction process has caused severe environmental problems and such treatment conventionally requires a large amount of investment for anti-pollution facilities. Recently, various methods have been proposed for the recovery of terephthalic acid from the polyester weight-reduction waste water, since said recovery may be useful not only in reusing the resources, but also in abating the enviromental problems.
The conventional processes of manufacturing and recovering terephthalic acid are as follows, using polyester weight-reduction waste water:
In Japanese Patent Unexamined Publication No. 50-104,276, terephthalic acid alkali salt was neutralized with sulfuric acid to give terephthalic acid.
In Japanese Patent Unexamined Publication No. 60-19,784, ultra-filtration was introduced to remove impurities and then, terephthalic acid was obtained through neutralization with sulfuric acid.
In Japanese Patent Unexamined Publication No. 60-163,843, alkali waste water was centrifuged and neutralized with sulfuric acid to give terephthalic acid.
In Japanese Patent Unexamined Publication No. 60-216,884, alkali waste water was passed through ion-exchange membrane to give terephthalic acid.
In Japanese Patent Unexamined Publication No. 60-233,033, alkali waste water was neutralized at 120.degree. C. and 1.7 atm to give terephthalic acid.
In Japanese Patent Unexamined Publication No. 61-43,139, alkali waste water with low concentration was adjusted to pH 5 to 6 and pH 4 two times to deposit terephthalic acid and then, centrifuged to recover terephthalic acid.
In Japanese Patent Unexamined Publication No. 61-43,140, hydrochloric acid was added to alkali waste water until the pH of the solution became 5.4 and treated with activated charcoal. Then, the solution was again added with hydrochloric acid until the pH of the solution became 2, and terephthalic acid was obtained.
In German Patent No. 2,508,819, alkali waste water was treated with sulfuric acid at 60 to 94.degree. C. to give terephthalic acid.
In U.S. Pat No. 5,210,292, alkali waste water was adjusted to pH 6 to 9, cooled to remove sodium sulfate and then, this material was again adjusted with sulfuric acid to pH 2 to 4 to give terephthalic acid.
These reported methods as aforementioned have also several problems as follows; a) a majority of their reactions was conducted under the severe conditions, b) excess water designed to enhance the purity of terephthalic acid, produced a considerable amount of waste water, which is economically unfeasible, c) terephthalic acid was not easily separated, through the method of modulating pH, from sodium salt generated from the neutralization reaction of alkali waste water, and d) said reported methods, which failed to illustrate some methods of removing impurities and purity-monitoring methods. Especially, these reported methods failed to remove sodium salt, by-product of acid neutralization, and it may be responsible for producing another environmental pollutants. In addition, in a process of filtering terephthalic acid as a final recovery step, the particle size of terephthalic acid should be sufficiently enlarged because small particles of terephthalic acid cause insufficient separation into solids and liquids which is responsible for reduction of recovery rate, and also make it difficult to perform the drying process.
Thus, the terephthalic acid obtained by the reported methods was not sufficient for reuse owing to reduction in recovery rate and purity and further, the purity of terephthalic acid in said reported methods was analyzed by general method instead of the method to analyze the quality of terephthalic acid. In this context, said reported methods are technically and economically unfavorable for commercialization.